Moisture content of materials is a key parameter in many research and industrial applications, including the food and agriculture-related industries. The most widely used standard techniques for moisture content determination are oven drying techniques. These techniques are based on drying samples under specific conditions, such as temperature and time, depending on the material. Besides being energy and time consuming, in some instances the representative character of the samples might be questionable compared to the whole volume or mass of material under consideration. Moreover, most industrial processes are highly automated and require real-time, on-line measurement of the moisture content.
Electromagnetic wave-interaction-based techniques meet this requirement and provide a tool for continuous measurement. In this way, by averaging, a better estimate of the moisture content can be achieved. Among these techniques, free-space microwave measurement techniques have the advantages of being nondestructive and contactless. Therefore, they are suitable for on-line, real-time monitoring and control. However, with particulate materials, bulk density fluctuations, as material moves on a conveyor belt or flows through a chute or pipe, can produce significant errors in moisture content determination. It is possible to reduce these fluctuations by mechanical means by keeping the layer thickness constant or by using a vibrator to maintain an average density. However, this still produces unpredictable errors in moisture content because of the density effect. In this instance, the density has to be determined by a separate method, such as gamma-ray attenuation or weighing. A separate density measurement is always an additional cost with more technical complications in the design and implementation of the measuring system.
A better alternative is to identify empirically or define theoretically density-independent functions exclusively dependent on moisture content. From an industrial perspective, the concept of density independence is a convenient solution for a cost-effective sensor that fulfills specific requirements. Therefore, the density-independent functions should be easy to manipulate for moisture content computation and tolerate instabilities produced by the measuring system and the immediate environment as well. Most of the transmission systems for moisture content determination are based on the principle of two-parameter measurement, namely the attenuation ΔA and phase shift ΔΦ and use of the ratio:ΔA/ΔΦas a density-independent function (Kraszewski et al., J. Microwave Power, Volume 12 (3), 241-252, 1977). This ratio was identified, empirically and can be used only in a transmission configuration over a limited moisture content range (Menke et al., IEEE MTT-S International Microwave Symposium Digest, Volume 3, 1415-1418, 1996).
To generalize the concept of density independence, the function has to be expressed in terms of universal entities such as the dielectric properties. The dielectric properties of materials are intrinsic properties usually expressed as the relative complex permittivity:∈=∈′−j∈″where ∈′ is the dielectric constant, which represents the ability of a material to store electric energy, and ∈″ is the loss factor, which represents the loss of electric-field energy in the material. Another parameter often used to describe the amount of loss is the loss tangent, tan δ, defined as the ratio:∈″/∈′.The dielectric constant and loss factor, as well as the loss tangent, of moist substances are generally dependent on frequency, temperature, density, and moisture content. The influence of these variables on the relative complex permittivity has been explored and reported for many materials (Nelson et al., J. Agric. Eng. Res., Volume 21, 181-192, 1976; Kent, J. Microwave Power, Volume 12 (4), 341-345, 1977; Meyer et al., IEEE Trans. Microwave Theory Techn., Volume MTT-29 (7), 732-739, 1981; Nelson, Cereal Chemistry, Volume 58 (6), 487-492, 1981; Nelson, J. Microwave Power, Volume 18 (2), 143-153, 1983; Kress-Rogers et al., J. Food Eng., Volume 6, 345-376, 1987; Kraszewski et al., J. Microwave Power and Electromagn. Energy, Volume 31 (3), 135-141, 1996; Trabelsi et al., Microwave Power and Electromagn. Energy, Volume 32 (3), 188-194, 1997; Trabelsi et al., Meas. Sci. and Technol. 14, 589-600, 2003).
Present state-of-the-art microwave moisture measurement systems attempt to eliminate density fluctuation effects by secondary measurements of density with gamma radiation gauges or other techniques, or by taking the ratio of attenuation and phase-shift in microwave measurements. These techniques limit the errors in moisture content determination attributable to fluctuations in bulk density, but seldom do they eliminate the density effects entirely. Also, secondary measurements of density complicate measurement systems and increase their consequent costs.
For grains, moisture is an important factor affecting price paid for grain. Therefore, moisture content must be determined whenever grain is traded. If moisture content is too high at the time of harvest, the grain kernels can be damaged in the mechanical harvesting process, leaving them more susceptible to infection by fungi. If they are stored at moisture contents too high for the prevailing environment, they can spoil because of the action of microorganisms, and the value is degraded or completely lost for human and animal consumption. Reference methods for determining moisture in grain generally require oven drying at specified temperatures following prescribed laboratory procedures (ASAE, 2000, ASAE S352.2, American Society of Agricultural Engineers, St. Joseph, Mich., pp. 563), or chemical titration methods, which are also laboratory procedures. Therefore, these methods are too slow and tedious for practical use in the grain trade. Electrical measurement methods have been developed that depend on correlations between the electrical properties of the grain and moisture content (Nelson, Transactions of the ASAE, volume 8 (1), 38-48, 1965; Nelson, Journal of Microwave Power, volume 121(1), 67-72, 1977). Electrical meters for grain moisture determination have evolved over the past century (Nelson, IEEE Transactions on Electrical Insulation, Volume 26(5), 845-869, 1991), and grain moisture meters in the United States today are predominantly those operating from 1 to 20 MHz that sense the dielectric properties (relative permittivity) of the grain samples. These instruments, although troubled with inconsistency at moisture contents above 20% to 25% moisture content, perform reasonably well, and calibrations are maintained by the manufacturers for many different grain and seed commodities. Moisture meters used in the trade require static samples, and corrections are made for variations in temperature and bulk density of grain samples. Needs have long been recognized for moisture sensing instruments for applications with moving grain, and efforts have been devoted to developing RF dielectric type moisture monitoring instruments. The need for moisture monitoring on combines as grain is harvested has spurred such development. Modern agriculture, involving precision farming, which generally implies yield mapping with the application of global positioning systems and grain mass flow monitoring, requires reliable moisture monitoring also, because yield data need to be based on a specific moisture content. Fluctuation in bulk density when grain is flowing causes errors in moisture readings unless some compensation is provided for bulk density changes. Thus, moisture monitoring system design must provide some means for minimizing bulk density variation.
Research on sensing moisture content in grain by microwave measurements has indicated two important advantages for microwave frequencies. The inconsistency of moisture measurements by instruments operating in the high-frequency range may be due, in part, to the influence of ionic conduction on the measured dielectric properties at high moisture levels. At microwave frequencies, the influence of ionic conduction is negligible, and better correlations between permittivity and moisture content can be expected. In addition, techniques for density-independent moisture sensing in granular materials have been reported for measurements at microwave frequencies (Kraszewski, Journal of Microwave Power, Volume 23(4), 236-246, 1988; Kraszewski, Journal of Agricultural Engineering Research, Volume 71, 227-237, 1998; Kraszewski and Kulinski, IEEE Transactions on Industrial Electronics and Control Instrumentation, Volume 23(4), 364-370, 1976; Kraszewski et al., Journal of Agricultural Engineering Research, Volume 72, 27-35, 1999; Trabelsi et al, Electronics Letters, Volume 33(10), 874-876, 1997; Trabelsi et al, IEEE Transactions on Instrumentation and Measurement, Volume 47(1), 127-132, 1998a; Trabelsi and Nelson, Measurement Science and Technology, Volume 12, 2192-2197, 2001b). While various methods have been developed for measurement of properties of different materials, there remains a need in the art for a method for simultaneous, independent real-time measurements of bulk density and moisture content of materials. The present invention provides a method which is different from prior art methods and solves some of the problems associated with the measurement of density and moisture content of bulk materials.